A belt tensioner having the above-mentioned features is known from US 2006/0097506 A1. The tensioner drive, which has a linearly movable piston, is situated eccentrically and at a distance to the belt shaft of the belt retractor. A metal strip is guided transversely over the movement path of the piston as the transference means for the linear movement of the piston into a rotational movement of the belt shaft and is stopped having one end fixed on a housing part, while the other loose end of the metal strip is guided partially wrapped around an extension of the belt shaft and guided further from here in a linear course as a reserve for the movement of the metal strip when the tensioner drive is activated. The metal strip is supported against a housing wall which encloses the shaft extension in the area of its partial wrap around the shaft extension before the tensioner drive is triggered, so that the metal strip forms a gap or clearance to the shaft extension. For better connection of the shaft extension to the metal strip during the tensioning procedure caused by the movement of the metal strip, the shaft extension has radially projecting pins, which engage in one or more perforations situated correspondingly in the metal strip. If the pyrotechnically actuated tensioner drive is triggered, the resulting gases drive the piston, which engages the metal strip guided transversely over its movement path and thus initiates a traction movement on the free end of the metal strip, which forms a winding reserve. This traction movement has the result that the metal strip is now wrapped around the extension of the belt shaft while overcoming the initially set clearance and sets the belt shaft into rotation because of the formfitting engagement with the shaft extension.
The known belt tensioner has the disadvantage that because of the winding reserve housed using a linear course, a correspondingly larger space is required for the belt tensioner.
The invention is therefore based on the object of implementing a belt tensioner having the features according to the species cited at the beginning as more compact in its construction.
The achievement of this object, including advantageous embodiments and refinements of the invention, results from the contents of the patent claims which are appended to this description.